This invention relates to improvements in a paper feeding apparatus used in an electrostatic copier or a document image reader in which the lowermost document in a document stack placed on a document stacker can be separated one by one and conveyed to the exposure position on a platen glass.
A recirculating document handler (RDH) and an automatic document feeder (ADF) are used as an automatic document feeder in which a plurality of documents are stacked on a document stacker and the documents are conveyed onto a platen glass of a copier. The paper separating efficiency of a document feeding unit is very important in this kind of apparatus.
The bottom conveyance type of document feeding apparatus is conventionally used as it is excellent in the paper separating efficiency. In Japanese Patent Application O.P.I Publication Nos. 69637/1983 and 76775/1976, this type of document feeding apparatus is disclosed. This apparatus is composed in such a manner that: the lowermost document stacked on a document stacker is separated from the stack and conveyed to the processing unit one by one; and the document is returned to the document stacker or a delivery stacker after the document is exposed.
A typical document feeder of a recirculating document handler (RDH) which realizes the bottom conveyance type of document feeder mentioned above, is the bottom conveyance and upper piling type of document feeder which is composed in such a manner that: a document feed opening is provided to the lower edge-portion of a document feeding unit; a document which is sent from the first document feeding unit close to the document feeding port, is guided from the second document feeding unit through the document feeding passage onto the upper surface of the platen glass of a copier; the document is moved on the platen glass to the exposure position by the motion of a conveyance belt provided on a platen glass; when the document is placed at the exposure position, an optical exposure system is reciprocated in order to expose the document; the exposed document is moved by the motion of the conveyance belt; and the document is conveyed through the recirculating passage and stacked on the uppermost position of the document stack placed on the document stacker.
In the conventional document feeder described above, the first document feeding unit is composed of: a document feeding belt which feeds the lowermost document of the document stack placed on the document feeding position; and a stop roller which comes into contact with the document feeding belt with pressure in order to prevent double feeding of documents. However, in the case of the document feeding apparatus described above, when the documents located at the regular position on the document stacker, are pushed by a push belt to the document feeding position, a plurality of documents are squeezed into the wedge-shaped portion formed by a document feeding belt and a stop roller and furthermore the documents enter into the nip portion.
Furthermore, as the above-mentioned stop roller comes into contact with the above-mentioned document feeding belt with pressure, the front side of the document and the reverse side of another document are rubbed with each other, so that the document surfaces are stained and the image on the document is damaged.
In order to solve the problems described above, the separating document feed system has been proposed in which the suction force or the blowing force by air is used.
The first type of the system was disclosed by the U.S. Pat. No. 4,345,751, which is the rotary suction document separating type of document feeding apparatus in which an rotating vacuum cylinder and the document conveyance unit are combined. This system is characterized in that: a vacuum suction cylinder is provided close to the tip of a document stack located on a document stacker; only the lowermost document of the stack is separated from the document stack by the suction force of the vacuum suction cylinder; and the separated document is adhered to the curved surface of the cylinder and sent downward so that the document can be transfered to the following conveyance unit. After the document is conveyed by the vacuum cylinder, the opening portion of the cylinder is returned to the position right below the document stack.
The cylinder unit of the document feeding apparatus of this proposal must be provided with a mechanism which is characterized in that: when a document is delivered, the vacuum suction is turned on; and when the cylinder is returned, the vacuum suction is turned off.
Furthermore, a strong vacuum suction unit by which the heavy static pressure can be generated, is necessary in order to increase the document separation force, so that it causes such problems that: the noise is increased; a wide space is necessary to install the unit; and the manufacturing cost of the document feed unit is increased.
In this rotating vacuum cylinder type of document feeding apparatus, a pipe made from aluminum alloy is used as a suction and conveyance surface. When the coefficient (.mu.) of friction between the aluminum alloy surface and the document surface is 0.3 to 0.5 and the coefficient of friction between the document surfaces is 1.0, the force necessary to pull out a document from a document stack composed of documents of A3 size, is about 1 kg The force needed to pull out a document from a document stack is determined by the area of the opening of the vacuum cylinder, the static suction pressure, and the coefficient (.mu.) of surface friction. When a strong force is given to a document in order to pull it out from a stack, the front side of one document and the reverse side of the other document are rubbed, which causes such a problem that the surfaces of documents are stained and damaged, so that the quality of images is deteriorated.
The above-mentioned vacuum cylinder and the conveyance roller must be eccentrically placed to the vacuum belt (the negative pressure belt) and their insides must be divided into two in the case of a document feeding apparatus which is composed in such a structure that: a plurality of throughholes are provided to the endless belt which conveys a document from the document stack to the platen glass; and negative pressure is activated to the document through the holes on the belt.
In the case of a plurality of endless negative pressure belt, the irregularity of speed is liable to occur among the belts. As a result, the conveyed document is sometimes deformed. Unless the deformation of a document is eliminated before copying, the document image can not be formed on a recording paper correctly. Furthermore, the structure and motion of this type of document feeding apparatus are complicated and especially the rotating mechanism of the suction drum is complicated.
Further, U.S. Pat. Nos. 4,284,270, 4,324,395, 4,411,417, and the like disclose another conventional document feeding system, which is called the air-knife document separating system.
In this document feeding system, the lowermost document is conveyed by a vacuum belt in such a manner that: the lowermost document of a document stack is sucked by a vacuum suction belt having a protruded portion in the middle so that a space can be made between the document and the document stack; air is blown into the space so that the document stack can be floated by air pressure; and the lowermost document can be pulled out from the stack.
When the document has been delivered from the above-described vacuum suction belt to the following conveyance roller, the vacuum suction must be stopped until the trailing end of the document passes through the vacuum suction belt. The reason to stop the vacuum suction is that: if the vacuum suction is continued after the document is delivered to the conveyance roller, the document is rubbed by the vacuum belt or the document is pulled by the vacuum suction belt. Consequently, it is necessary for the vacuum suction belt unit to be turned on or off every time a document is fed. For that reason, consideration must be given to the suction preparing time which is defined as the time (about some hundreds millisecond) necessary to start the vacuum suction after the vacuum unit is turned on. The suction preparing time is 10 times longer than that of the conventional friction separating type of document feeder, wherein in the case of the conventional friction separating type of document feeder, this suction preparing time is 30 to 50 ms which is the same as the response time of a magnetic clutch. As a result, the response lag of the air-knife separating system is 10 times larger than that of the conventional magnetic clutch system. For that reason, the air-knife separating system is inferior from the view point of the follow-up ability at a high speed, so that it is not suitable for high speed document separation and conveyance. Accordingly, these types of document feeding apparatuses have such problems that: (1) the shape of the document stacker surface is not simple, so that the shape of the suction box can not cope with various sizes of documents; (2) as the document is sucked by an air gap, the lead time is necessary, so that these types of document feeding apparatuses are not suitable for high speed document feeding; (3) as a special blower is necessary for these types of document feeding apparatuses, the control is complicated and the cost is increased.